Next generation cellular networks are expected to be characterized by their extreme density. The current paradigm of having isolated cells (less than 4% overlap) operated by a single, centrally placed cell tower, equipped with dedicated spectrum and isolated from its neighbours by guard bands, will be replaced by a dense network of Distributed Antenna Systems and Remote Radio Heads (hereinafter referred as RRH/‘network nodes’), coordinating with each other to share a large coverage region using a shared frequency band.
Thus, the new paradigm offers unprecedented benefits, in terms of instantaneous throughput, energy conservation and edge-of-cell performance. However, it will also have enormous associated challenges with respect to user scheduling and spectrum sharing; especially in cross-node interference.
Heterogenous networks are one such type of network existing in new paradigm. They consist of a mix of macro and pico/femto base stations or small-cells operating in close proximity. Recent innovations in the area of inter-cell association allow these nodes to coordinate closely with each other in real-time to increase system capacity and user service. As networks become more and more squeezed and the reuse distance drops, network operators have demanded additional tools to manage the interference load which limits system performance, especially at the cell edge. In response, the 3GPP has standardized a number of mechanisms for achieving this under the term Inter-cell Interference Coordination (ICIC), followed by enhanced (eICIC) and further enhanced ICIC.
FIG. 1 illustrates Inter-cell Interference Coordination (ICIC) deployment for heterogeneous networks. The network comprises of one macro-cell and several small-cells. Each small-cell shares a common frequency with the macro-cell. There are users in the boundary zone between the macro-cell and any given small-cell, which can receive wireless transmissions either from the macro-cell or by the small-cell. Particular users will be scheduled to use the small-cell and others will be scheduled to use the macro-cell. Since the macro is resource limited, it would like to let the small-cell handle as many users as possible. The small-cells, on the other hand are power limited. Thus, small cells are not able to interfere with each other, but each small-cell can and does interfere with (and are subject to interference from) the macro-cell.
At each point of time, the macro-cell and the small-cell have to schedule transmissions to the UEs associated with each of them. If the macro-cell and the small-cell transmit on the same resource in the same time-slot, there will be interference.
In the standard ICIC/eICIC scenario, the macro-cell will coordinate with the small-cells by creating dedicated time-gaps, called ABS (Almost Blank Subframes) or RBS (Reduced Power Subframes) where the macro-cell either transmits no data or backs-off its transmit power significantly. This gives an opportunity for the small-cells to transmit. However, in these frames the UEs scheduled to the macro do not get data, and in the other sub-frames, it is the UEs scheduled with the small-cells which do not receive data. The ABS/RBS frames count as loss of capacity to the macro-cells (though the capacity saved in one sector could potentially be used in other sectors or destinations).
Coordinated Multipoint (COMP) is a technology based on the ability for multiple endpoints to coordinate as part of a common MIMO (Multiple-Input and Multiple-Output) transmission. The efficiency of MIMO is increased when the number of antennae used are larger and the spatial separation is high. When multiple transmitters or receivers coordinate implicitly or explicitly with each other (using, for example, opportunistic scheduling) in order to use multi-user MIMO, this becomes a case of Coordinated Multipoint (CoMP). Theoretically CoMP can achieve significant gains in throughput by utilizing the statistical diversity of the wireless channels. In reality, there are significant challenges in terms of inter-node coordination.
FIG. 2 shows different forms of multi-user MIMO transmission using Coordinated Multipoint such as Broadcast transmission and Joint Transmission. In Joint Transmission, where one cell (either the macro-cell or the small-cell) could simultaneously transmit to multiple UEs.
In FIG. 3, shows an urban network deployment comprising one macro-cell and several small-cells. In one possible scheme, a pair of the UEs (ues and uem) are treated as a single unit and, based on their measured/reported channel characteristics, the macro-cell decides whether they should be broadcast from the small-cell or itself. This determination could be done for all the UEs which are eligible, on a pair by pair basis. This scheme has the downside of leaving some small cells idle, but has the upside that it mitigates the inter-cell interference, as described below. The problem is that from any given network node, the individual members of the targeted UEs will have different channel conditions, so it will be hard to implement scheduling in a fair yet efficient manner.
In another scheme, which address similar problems include Block diagonalization (BD or zero-forming). Block diagonalization is a technique allows a single transmitter to transmit to multiple receivers simultaneously, without cross-receiver interference. The fundamental principle of BD is to choose orthogonal pre-coding matrices, effectively making each receivers data stream invisible to the others. However, in the case of block-diagonalization, orthogonalization is achieved by choosing a pre-coding matrix which is orthonormal to the co-resident UEs; it does not take the target UEs channel matrix into account. Further, BD is unable to utilize additional antenna (Nt>Nr). This means that is unable to utilize modern networks with RRH and massive-MIMO capabilities.
Hence, there is a need to have a system and method that can overcome the above stated problems and provides a system and method with enhanced scheduling and simultaneously maximizing network capacity and mitigating cross-cell interference.